Transformer with a carefully adjusted primary inductance



Jan. 18, 1966 w. E. w.

JACOB 3,230,488

TRANSFORMER WITH A CAREFULLY ADJUSTED PRIMARY INDUCTANGE Filed Sept.

j INVENTOR.

H-rraRA/e'rs 3 230 488 TRANSFORMER WITH A iZAREFULLY ADJUSTED PRIMARY INDUCTANCE Walter Emil Wiiheim Jacob, Hagersten, Sweden, assignor to Telefonaktiebolaget L M Ericsson, Stockholm, Sweden, a corporation of Sweden Filed Sept. 17, 1962, Ser. No. 224,036 Claims priority, application Sweden, Nov. 1, 1961, 10,871/ 61 2 Claims. (Ci. 336-17tl) The present invention refers to a transformer with a carefully adjusted primary inductance particularly intended for use in electronic circuitry. The many applications in the electronics field, for instance for controlling rapid electronic transistor contacts, pulse transformers with carefully adjusted primary inductance are wanted. Another demand made upon such transformers is that the leakage between the primary and the secondary winding must be as small as possible in order to obtain short rise times for the pulse voltage on the secondary side, that is, the leakage inductance must be negligible in comparison with the primary inductance. A third demand is that the transformation ratio between the primary and the secondary side may be substantially different from 1:1.

In practice it has been found to be diificult to satisfy the aforementioned demands at the same time. These difficulties are briefly described below. The inductance L of a transformer winding can be Written L=)\-n where A is the inductance factor of the used core type and n is the number of turns on the core in question. For a wanted transformation ratio p/ q between the primary and the secondary side of the transformer, the primary number of turns may be a multiple of p. If the number of turns on the primary side is n+p the primary inductance will be L \(n+p) Let it be assumed that the inductance is too small with 771 turns but too large with n-j-p turns. The divergence a of the inductance, written in percent, is obtained from the relation from which relation the least number of necessary turns required for a certain percental divergence a can be obtained. This divergence, which is an expression for the tolerance demand, causes in most cases the primary inductance to be substantially larger than wanted. It has been attempted to avoid such excessive inductance, for instance by diminishing the inductance factor A, that is by using a smaller core or/and arranging an air gapin the core. However, there are disadvantages to this approach. The number of available core types is severely limited, which entails inadequate variation possibilities, and the arrangement of a variable air gap in small core types presents difficult mechanical problems. As generally an airgapless toroidal or tubular core is wanted, attempts have been made to solve the problem by using compressed powder cores with low a-value. However, this approach also has its drawbacks, partly due to the limited number of available core types, which restricts the possibilities of obtaining a wanted a-value, partly due to the fact that low a-values give rise to large leakage inductance. This could be avoided by a suitable winding system, but then a capacitance, which in many cases is not negligible, is introduced between the primary and the secondary windmg.

The purpose of the invention is to eliminate the drawbacks mentioned in the foregoing. A transformer according to the invention is characterized by the fact that its windings are wound with the desired transformation ratio on a core of a material with a ,u-value so high that the leakage inductance is quite negligible, whereby the 3,23,438 Patented Jan. 18, 1966 primary winding of the transformer is wound with a number of turns, which with a whole number multiple exceeds the number of turns which could be sufficient for obtaining the desired primary inductance. The primary winding is hunted by an inductance by means of which the primary inductance can be accurately adjusted.

The invention will be further described in connection with the attached drawing, where FIG. 1 schematically shows a transformer according to the invention, FIG. 2 shows an end view of a transformer structure according to the invention, having a cylinder-shaped core, FIG. 3 shows a section along line A-A of FIG. 2, and FIG. 4 shows an example of an adjustable inductance constituting an integral part of the transformer according to the invention.

The transformer according to FIG. 1 comprises a core K without an air gap, a primary winding L1 and a secondary winding L2. The core K is formed of a material with so high a [.t-Villllfi that the leakage inductance is quite negligible compared with the primary inductance of the transformer. The number of turns of the windings L1 and L2 are so chosen that the desired transformation ratio p/q is obtained. The primary winding L1 is wound with a number of turns mp, which exceeds the number of turns, which theoretically should be sufficient to obtain a desired primary inductance and is shunted by an inductance coil D wound upon core K to provide an inductance D shunting the primary winding L1 and so arranged that the primary inductance of the transformer can be adjusted to the desired value.

With the transformer as described above, a negligible leakage inductance is obtained owing to the strong coupling between the primary and secondary windings as a result of the high ,u-value and a low capacitance between the primary and secondary windings owing to the relative low number of turns of these windings. In such a transformer, the value of the inductance D and the choice of the number of turns and of the core material are not as limited as with the known transformers previously described, and the leakage has not the significant influence as with the known transformers referred to herein. Con sequently it is possible to choose a core material with a low ,u-value for the inductance D The inductance value L of the inductance D is so adjusted that the relation where L is the desired primary inductance and L is the inductance value of the primary winding L1.

The arrangement according to FIGS. 2 and 3 consists of a transformer with a cylinder shaped core K, on which two sector shaped primary and secondary windings L1 and L2 respectively are wound. About the core K and the windings L1 and L2, perpendicularly to their longitudinal axes, the inductance coil D is wound, whereby its number of turns is so chosen that the desired primary inductance is obtained between the terminals 1 and 2 of the windings L1, across which terminals the inductance coil D is connected at 3 and 4. With the same core a high x-value is obtained for the windings L1 and L2 but a low t-value for the inductance coil D, dependent on the different fluxes involved.

The inductance value of the inductance coil D can, however, be varied in another way, for instance according to FIG. 4 which shows a coil D arranged on a core having two centrally apertured parts K1 and K2, in which is arranged a setting screw S which can be pushed more or less into the core and thereby influence the size of said inductance value.

In connection with FIG. 1 a numerical example of data will now be given for a transformer according to the invention. It is assumed that the desired resulting primary inductance L is 50 h, that the transformation ratio 2/ q of the transformer is 3:1, that the number of turns of the primary winding L1 is twelve; that is the number of turns of the secondary winding L2 is four and that the inductance of the primary winding is 90 p.11. The inductance factor A of the transformer core K is now According to the already mentioned relation a value of L =l12 ,uh. is then obtained, and for this inductance value a number of turns is required, if for the inductance D, a core material with the inductance factor A:.025 [LIL/17,218 used.

I claim:

1. A transformer comprising a hollow cylindrical core of high permeability material, a primary winding adapted to receive an input signal, said primary winding being toroidally wound about said hollow cylindrical core, a secondary winding for transmitting an output signal in response to an input signal received by said primary winding, said secondary winding being toroidally wound around said hollow cylindrical core, a shunt inductance winding circumferentially wound about said hollow cylindrical core, and means for electrically connecting said primary winding in parallel with said shunt inductance winding, whereby the effective value of the inductance of said primary winding is reduced in accordance with the value of the inductance of said shunt inductance winding.

2. A transformer comprising a hollow cylindrical core of high permeability material, said hollow cylindrical core having an outer side wall portion, a primary winding having a first given number of turns toroidally wound about a first portion of said hollow cylindrical core, a secondary winding having a second given number of turns toroidally wound about a second portion of said hollow cylindrical core so that said transformer has at least a primary inductance having a value greater than a given value, a shunt inductance coil circumferentially wound about the outer side wall portion of said hollow cylindrical core so that said hollow cylindrical core and portions of said primary and secondary windings provide a core of low permeability for said shunt inductance coil, said shunt inductance coil having a number of turns to provide a given value of shunt inductance, and means for connecting said shunt inductance coil electrically in parallel with said primary winding so that the elfective value of the primary inductance is reduced to said given value.

References Cited by the Examiner UNITED STATES PATENTS 2,190,448 2/1940 Freygang 336229 X 2,592,721 4/1952 Mott 336-210 X 2,786,983 3/1957 Hill 33683 2,899,654 8/1959 Geiser 336-136 X 2,966,704 1/1961 OBrian et al 33683 X ROBERT K. SCHAEFER, Primary Examiner.

JOHN F. BURNS, LARAMIE E. ASKIN, WALTER M.

ASBURY, Assistant Examiners. 

1. A TRANSFORMER COMPRISING A HOLLOW CYLINDRICAL CORE OF HIGH PERMEABILITY MATERIAL, A PRIMARY WINDING ADAPTED TO RECEIVE AN INPUT SIGNAL, SAID PRIMARY WINDING BEING TOROIDALLY WOUND ABOUT SAID HOLLOW CYLINDRICAL CORE, A SECONDARY WINDING FOR TRANSMITTING AN OUTPUT SIGNAL IN RESPONSE TO AN INPUT SIGNAL RECEIVED BY SAID PRIMARY WINDING, SAID SECONDARY WINDING BEING TOROIDALLY WOUND AROUND SAID HOLLOW CYLINDRICAL CORE, A SHUNT INDUCTANCE WINDING CIRCUMFERENTIALLY WOUND ABOUT SAID HOLLOW CYLINDRICAL CORE, AND MEANS FOR ELECTRICALLY CONNECTING SAID PRIMARY WINDING IN PARALLEL WITH SAID SHUNT INDUCTANCE WINDING, WHEREBY THE EFFECTIVE VALUE OF THE INDUCTANCE OF SAID PRIMARY WINDING IS REDUCED IN ACCORDANCE WITH THE VALUE OF THE INDUCTANCE OF SAID SHUNT INDUCTANCE WINDING. 